This invention pertains to a laminate base material of aromatic polyamide fiber non-woven fabric and to a method of producing the same. Also, this invention provides a laminate having a base material of aromatic polyamide fiber non-woven fabric constructed in accordance with the invention and suitably available for an insulating board for a printed wiring board on which leadless chip parts such as resistors, ICs and so on are surface-mounted.
Of late, in case that electronic parts such as resistors, ICs and so on are mounted on a printed wiring board which is assembled in an electronic device or instrument, the electronic parts have tended to be chipped and mounted on the printed wiring board by a surface-mount system. The surface-mount system is a preferable one because the electronic parts are compacted and lightened and have a high density.
In case that the leadless chip parts are surface-mounted on the printed wiring board, an insulation board of the printed wiring board should be so considered to have a coefficient of thermal expansion matched to that (2-7.times.10.sup.-6 .degree. C.) of the leadless chip parts as much as possible. This is because if there is much difference between the coefficients of thermal expansion of the insulation board and the electronic parts, then there tends to occur a crack in soldered connections of the leadless chip parts due to a repetition of thermal shock cycle.
In view of this, a base material of non-woven fabric of aromatic polyamide fibers having a negative coefficient of thermal expansion has been considered to be used for a laminate for a printed wiring board which may have a metal foil or foils clad thereto. Such a non-woven fabric may be formed by paper-making aromatic polyamide fibers such as p-phenylene-terephthalamide fibers or p-phenylene-diphenylether-terephthalamide fibers and binding the fibers to each other by a resin binder to form a paper. The laminate is formed by heating and pressing the non-woven fabric base material impregnated with thermosetting resin. Normally, a metal foil or foils may be heated and pressed onto the laminate so that they are integrated there and form a metal clad laminate.
However, the prior laminate having the base material of conventional aromatic polyamide fiber non-woven fabric disadvantageously tends to get warpage or twist after formed or worked into the printed wiring board. Upon studying the causes due to which the warpage or twist of the laminate occurs, it has been found that the aromatic polyamide fibers of the base material are unevenly deformed due to fluidity of the thermosetting resin which is molten and moved by heat and pressure applied thereto on forming the laminate.
Since a melting temperature of the thermosetting resin with which the base material is impregnated and which is normally 80 to 140.degree. C. is near or exceeds a glass transition temperature of the resin binder, the latter is so softened that the aromatic polyamide fibers are released from being bonded to each other.
When the leadless chip parts are soldered by the surface-mount system onto the printed wiring board which is formed by working the laminate having the base material of the conventional aromatic polyamide fibers to which may be clad a metal foil or foils, the temperature of 200.degree. C. or more will be applied to the base material of the laminate. At that time, the base material will be partially expanded and shrank, but neither evenly expanded nor shrank so that it will show uneven movement. It has been found that this causes the printed wiring board to have warpage or twist provided thereto.